Power amplification and efficiency of molecular lasers, decreases with increasing temperature of the laser gas. With rising temperatures the line width becomes larger, the excitation energy distributes among an increasing number of rotational lines, the number of deactivating collisions increases and the population of the laser end level increases by thermal excitation which results in a decrease in inversion of the individual transitions.
Methods have been developed to carry off the heat together with the laser gas by circulating and cooling the gas. Lasers appropriate for this method generally comprise an active region in which the gas is excited with a separate (downstream) or integrated optical resonator of the gas transport system with a built-in cooler and a pump. Because large volumes of heat are carried off, large gas volumes have to be transferred by pumping. These lasers are complex and expensive.
Conventional gas transport lasers use a high-power blower, e.g. a fan or a Roots pump, for rapid gas circulation. The pump is most often operatively associated with heat exchangers, both of which require separate housings, typically at least one cast iron housing for the pump.
Depending on the arrangement of the laser resonator and the direction of gas flow, a distinction is made between transverse-flow and axial flow lasers. In a transverse-flow laser, the gas flows generally perpendicular to the axis of the laser beam and the axis of the discharge. The flow of gas in an axial flow laser is along the axis of the laser beam and the discharge.
Disadvantages of transverse flow lasers include: production of a non-symetrical beam with poorer mode quality; numerous anodes and cathodes are utilized to produce the necessary multiple discharges; and lifetime problems.
In comparison, axial flow lasers produce better beam quality, are simplier to implement and produce a desired gaussian beam in the TEM.sub.00 mode which is very symmetric.
However, a strong and heavy Roots pump is usually necessary and is typically housed in a cast iron vessel. These pumps are designed primarily for evacuation purposes, are very bulky, have external shaft seals and complex seals for recirculating. Each side of the pump requires a heat exchanger which is generally disposed in a separate housing. This requires more seals, water fittings, vacuum flanges and are generally relatively complex mechanical configurations.
U.S. Pat. No. 4,321,558 discloses enclosing the working parts of a flowing gas laser within an airtight housing. A conventional Roots type blower is included, necessitating external shaft seals, as well as water fittings and vacuum flanges for the heat exchangers. The blower is not readily removable from the housing, is not a clean pump and thus subject to contamination, and is housed in a heavy and expensive cast iron housing. The heat exchangers require vacuum seals, heavy housings and water seals.
It would be an advancement in the art to provide a laser gas flow circulating system which utilizes a lightweight positive displacement pump with no external shaft seals, heat exchangers requiring no vacuum-tight seals and no water fittings. It would be a further advancement to provide such a gas flow circulating system in which contamination is minimized and one module is used to house a light weight displacement pump and the heat exchangers with simple ducting. This would provide for easy removal of the pump from the module without expensive disruption of relevant connections to the heat exchangers.